Electrophotographic imaging elements, processes and techniques have been extensively described in both the patent and other literature. Generally, electrophotographic processes employ an electrophotographic element which responds to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well known in the art can then be employed to produce a permanent record of the image.
One type of electrophotographic element particularly useful in electrophotographic processes comprises a multi-layer structure. Such an element is prepared by coating a layer of a photoconductive composition onto a film support previously overcoated with a layer of conducting material. In addition, an insulating or barrier layer is interposed between the conducting layer and the photoconductive layer.
The barrier layer serves a variety of functions in electrophotographic elements. It primarily reduces the charge leakage of an electrophotographic element in the absence of activating radiation. The latter phenomenon is generally referred to as "dark decay".
For electrophotographic microimaging applications or for very high quality one to one electrophotographic reproductions, it is important for the photoconductive layer to resist localized dielectric breakdown. Dielectric breakdown in a photoconductive layer results in small, discrete areas on the surface of the layer, about 5 to 30 .mu.m across, which appear light struck (white spots). Such discrete areas are unable to hold a surface charge. Most photoconductive layers having a high photodischarge sensitivity suffer from an excessively high density of dielectric breakdown spots.
Previously disclosed barrier layers for electrophotographic elements include aluminum oxide, polystyrene and cellulose nitrate. The problem is that these barrier layers have not been successful in minimizing the density of dielectric breakdown spots in high speed, highly sensitive photoconductive layers intended for microimaging or high quality electrophotographic reproduction.